Various processes can be used to produce polyethylene, including gas phase processes, solution processes, and slurry processes. In ethylene slurry polymerization processes, diluents such as hexane or isobutane may be used to dissolve the ethylene monomer, comonomers and hydrogen, and the monomer(s) are polymerized with a catalyst. Following polymerization, the polymer product formed is present as a slurry of polyethylene particles suspended in the liquid medium.
In exemplary multi-reactor cascade processes, shown e.g., in WIPO Pat. App. Pub. No. WO 2005/077992 A1 or WIPO Pat. App. Pub. No. WO 2012/028591 A1, the reactors can be operated in parallel or in series, and the types and amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal or multimodal polyethylene material. Such multimodal compositions are used in a variety of applications; e.g., WIPO Pat. App. Pub. No. WO 2012/069400 A1 discloses trimodal polyethylene compositions for blow moldings.
In processes to produce polyethylene, feeding comonomers and hydrogen along with ethylene as main monomer may allow to adapt the properties of the resulting ethylene polymer to the required values. Adding comonomer may reduce the density of the polyethylene and may impact the polymer's stiffness, toughness and stress crack resistance. Adding hydrogen may reduce the molecular weight and may impact the polymer's processability. In multi-reactor processes with different polymerization conditions in each reactor, an even more targeted adaption of the polymer properties may be possible. In multi-reactor processes, comonomer and hydrogen may be red to the polymerization reactor along with the ethylene; alternatively hydrogen and ethylene without comonomer may be fed to multi-reactor processes; alternatively still comonomer and ethylene without hydrogen may be fed to multi-reactor processes.
Various approaches have been used to control continuous manufacturing processes in polymerization plants. European Pat. App. No. EP 0 318 609 A1 discloses a process for producing propylene copolymers where the fraction of gas-phase produced copolymer is controlled. European Pat. App. No. EP 2 336 200 A1 discloses a process to manufacture an olefin polymer in suspension in a series of two polymerization reactors.
Disclosed herein are methods for controlling a slurry polymerization for the preparation of polyethylene. In particular disclosed herein are methods for the operation of the reactors in series to produce bimodal or multimodal polyethylene, with control of each reactor to provide a control of the polymer properties of the final powder.